(a) Field of the Invention
The present invention relates to a linear block copolymer and a process for preparing the same, and particularly to a linear block copolymer in which a conjugated diene/vinyl aromatic hydrocarbon copolymer block is located in a block copolymer and a process for preparing the same.
(b) Description of the Related Art
Vinyl aromatic hydrocarbon/conjugated diene block copolymers prepared using organolithium and hydrocarbon solvent differ in their microstructures according to a ratio of vinyl aromatic hydrocarbon and conjugated diene, and processing conditions and physical properties of resultant products also differ. Generally, a rubber phase block copolymer having vinyl aromatic hydrocarbon contents of 50% or less is prepared by sequentially polymerizing a vinyl aromatic hydrocarbon and a conjugated diene and then introducing a coupling reactant.
WO 95/12644 has disclosed using a halogenous silane compound as a coupling reactant, which can shorten polymerization time and obtain a copolymer having vinyl aromatic hydrocarbon polymer blocks of the same molecular weights, but a lithium compound produced in the coupling reaction as a by-product causes discoloration during processing. In order to prevent this, a vinyl aromatic hydrocarbon block and a conjugated diene block are sequentially prepared and then the vinyl aromatic hydrocarbon is polymerized again, thereby preparing a linear triblock copolymer.
A linear or star block copolymer having vinyl aromatic hydrocarbon contents of 60 to 80% has superior transparency and impact strength and thus is used for packaging containers, etc. The block copolymer is also used for compensating impact strength of general-purpose polystyrene, and it is preferable to use a block copolymer having vinyl aromatic hydrocarbon contents of 70 to 80% for improving impact strength of polystyrene and for maintaining transparency. Recently, a linear or star copolymer having vinyl aromatic hydrocarbon contents of 20 to 50% has been blended with general-purpose polystyrene. Particularly, because the linear vinyl aromatic hydrocarbon/conjugated diene block copolymer prepared by sequentially introducing monomers does not generate side reactions such as discoloration during the polymerization process, it is suitable for molded products requiring high transparency, and it has superior dispersibility when blended with other resins and thus is suitable for improving impact resistance of polystyrene and for maintaining transparency.
In addition, U.S. Pat. No. 6,140,433 has disclosed a block copolymer with polydispersity that can improve impact resistance of other resins by sequentially introducing monomers when preparing a vinyl aromatic hydrocarbon/conjugated diene block copolymer, by introducing an initiator twice and introducing a vinyl aromatic hydrocarbon/conjugated diene copolymer block. However, although this method can easily control physical properties and processibility of a product, it is required that an exact amount of initiator is introduced and that polymerization should proceed at a high temperature. Additionally, the copolymer may lower transparency of the product due to side reactions generated by conducting polymerization at a high temperature.
Accordingly, there is a need for a process that can improve transparency by introducing a vinyl aromatic hydrocarbon/conjugated diene copolymer block into a block copolymer, and that can polymerize at a comparatively low polymerization temperature by introducing an initiator once.